1. Field of the Invention
This invention relates to an improved hard coating material having toughness and slideability characteristics and also to sliding members such as internal combustion engine piston rings having a film of the hard coating material.
2. Description of the Related Art
In recent years, piston rings have been subject to harsh operating environments due to demands for increased engine output and reduced exhaust gas emissions. Many engines using present methods such as hard chromium plating and nitriding treatment for surface processing of sliding surfaces, are not durable enough to withstand this harsh environment. A hard film of materials such as TiN and CrN using physical vapor deposition process has been proposed. A thick film of CrN in particular has been employed to meet demands for engines with increased durability.
The CrN film shows superior wear resistance and scuffing resistance. This film is easy to mass produce and available in layers exceeding 30 .mu.m. However since the CrN film is brittle, excessive repetitive stress on the sliding surfaces during operation generate cracks, which may cause peeling.